Systematically administered opioids activate bulbospinal noradrenergic pathways. The resulting spinally released norepinephrine activates a 2-adrenoceptors on cholinergic neurons, causing ACh release and inhibition of spinal transmission of noxious sensory information. This study investigates the interaction between i.t. neostigmine and intravenous alfentanil in potentiating analygesia from intraspinal a-adrenergic agonist and from systemic opioids.